Trona, a mineral having the approximate composition Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O, is extensively mined in the United States from deposits located in southwestern Wyoming. These trona deposits are in the form of horizontally extended beds having thickness of from about 5 to about 15 feet located at a depth from about 800 to about 2000 feet below the surface. These trona beds usually lie between horizontally extending strata of soft shale. The overlying and underlying strata usually have compressive strength in the order of about 3700 psi and are substantially weaker than the trona bed which has a compressive strength of about 7500 psi.
Trona, on acount of its hardness (hardness index of about 2.5 to 3.0), high compressive and tensile strength (about 7500 and 500 psi respectively), brittleness and high density (about 134 lbs./cu.ft.) is very difficult to mine. Energy and forces required to remove trona from subterranean deposits by dry mining techniques are very high, with consequent high mining machine wear and shortened tool life. Further, mining of trona by usual mechanical methods creates large amounts of dust, which is very undesirable from a health and safety standpoint.
The above-described physical characteristics of trona result in particular problems in the longwall mining method. Longwall mining is practiced by driving substantially parallel entries into the trona bed, connecting the entries by at least one primary passage to define a main trona pillar to be mined, supporting the roof of the primary passage by means of movable roof supports or "choks", mining the exposed face of the trona pillar along the primary passage under the protection of the roof supports, removing the mined trona and advancing the roof supports so that these supports remain adjacent and parallel to the exposed face of the trona as it recedes. As the roof supports are moved toward the receding trona face, the unsupported roof behind it collapses and fills with "gob". Longwall mining of trona is described in commonly-assigned U.S. Pat. No. 3,778,108, the disclosure of which is hereby incorporated by reference.
As the trona is removed from the exposed face of the trona pillar along the primary passage, and before the roof supports are advanced toward the recently mined face, the roof directly overhead of the recently mined area is unsupported. In the southwestern Wyoming trona formation, where the overlying and underlying strata are substantially weaker than the trona bed, the unsupported roof and the floor immediately adjacent to the mined area tend to converge, the roof sinking under the weight of the overburden, and the floor heaving as a result of removal of downward pressure. As a result of this convergence, the layer of trona adjacent the face is subjected to concentrated compressive stresses which cause the layer adjacent the face to "buckle away from" the trona bed as the mining along the face of the longwall progresses, causing large "slabs" of trona to break away from the face ("slabbing") and to fall in the way of the mining machine, thereby interrupting the mining operation until these slabs are broken up and cleared away.
Such slabs of trona come in various sizes. They may be no more than one or two feet measured in their largest dimension, in which case they cause no problems, but their largest dimension may equal or even exceed about 20 feet. Slabs measuring more than about 2 to 4 feet in their largest dimension are difficult to handle by conventional longwall mining equipment, and slabs measuring more than about 4 to 5 feet in their largest dimension often require shutdown of the mining operation until they can be cleared away.
It is an object of the present invention to provide an improvement in dry mining of trona to facilitate removal of trona by conventional mechanical methods.
It is another object of the present invention to provide means for eliminating or substantially reducing such "slabbing" of trona in the longwall method of mining which creates "slabs" measuring more than about 4 feet in their largest dimension.